General Classification of Factors
For a systematic understanding of pharmacotherapy, all conditions capable of altering a drug's effect are divided into four logical groups:
- Properties of the substance itself — chemical structure, physicochemical parameters, and quantitative characteristics (doses, concentrations).
- Pharmaceutical factors — the chosen drug formulation and its manufacturing technology.
- Patient-related factors — current functional status, pathologies, sex, age, body weight, and genetics.
- Environmental and regimen factors — conditions of administration (repeated dosing, combinations), time of day, and environmental parameters.
Role of the Molecule and Drug Formulation
The fundamental profile of a drug is determined by its chemical structure. Substances within the same chemical class (e.g., benzodiazepines) produce a similar spectrum of effects. For a molecule to bind to a receptor, it must possess specific functional groups, the correct spatial orientation, and an appropriate size.
The drug formulation itself does not alter the mechanism of action, but it critically impacts pharmacokinetics. The formulation dictates the site and rate of absorption, and consequently, the time of onset and duration of the effect. Modern technologies enable the creation of extended-release formulations as well as the use of excipients to improve taste or reduce irritant effects.
Patient: Sex, Age, and Body Weight
Individual organism parameters require careful dose adjustments:
- Sex. Male sex hormones stimulate liver enzymes, which accelerates the metabolism of certain drugs (paracetamol, verapamil, benzodiazepines, ethanol) in men. Women have a higher risk of specific arrhythmias (torsades de pointes) when taking antiarrhythmic agents, and morphine may induce paradoxical excitation.
- Age. Two groups are most vulnerable. Neonates have poorly developed liver enzymes, reduced renal function, and a highly permeable blood-brain barrier (BBB), leading to increased toxicity (e.g., of chloramphenicol). In elderly individuals (over 60 years), absorption processes slow down, albumin levels drop, and blood flow decreases. Potent drugs are typically prescribed to them at 2/3 of the standard dose.
- Body weight. Drug concentration is inversely proportional to weight. In cachexia (wasting) or dehydration, the dose must be reduced by 1.5–2 times to avoid overdose.
Impact of Diseases and Genetics
Drug action directly depends on the baseline physiological and pathological state. For instance, acetylsalicylic acid lowers only elevated body temperature, while cardiac glycosides exhibit a positive inotropic effect specifically in heart failure. Local pathologies are also crucial: the efficacy of local anesthetics drops in an inflammatory focus, and that of sulfonamides drops in purulent wounds.
Systemic diseases (especially of the kidneys and liver) lead to cumulation (accumulation) of the substance. For example, in renal failure, the half-life of ampicillin increases from 1.3 to 20 hours.
Genetic factors are studied by pharmacogenetics. Gene mutations lead to enzymopathies—disruptions in the structure or function of enzymes. This slows down drug metabolism and triggers toxic reactions. The most striking example is idiosyncrasy (an atypical reaction). When glucose-6-phosphate dehydrogenase enzyme is defective, the administration of antimalarial drugs provokes massive erythrocyte hemolysis.